Sound on Programming the Atari VCS in Assembly

The Audio Registers

Mon, 01 Jan 0001 00:00:00 +0000

The Audio Registers#

The TIA gives you two independent audio channels, and that’s the whole orchestra — two voices, no more. Each channel is described by exactly three registers, and a sound is whatever you get by setting them and changing them over time.

Register	Width	Sets
`AUDC0` / `AUDC1`	4 bits	the waveform — which tone or noise shape this channel makes
`AUDF0` / `AUDF1`	5 bits	the frequency — a divisor, so 32 steps, higher value = lower pitch
`AUDV0` / `AUDV1`	4 bits	the volume — 0 (silent) to 15 (loudest)

The three combine in a fixed signal path: AUDC and AUDF feed a tone/noise generator, whose output is scaled by AUDV, and the two channels mix to the single output:

Tones, Noise & Pitch

Mon, 01 Jan 0001 00:00:00 +0000

Tones, Noise & Pitch#

Two registers shape what a channel sounds like: AUDC picks the waveform, and AUDF picks the pitch. Together they’re the source of both the VCS’s distinctive voice and its most infamous flaw — it can’t play in tune.

AUDC: pick a voice#

AUDC is a 4-bit selector (0–15) that routes the channel through different internal divider and polynomial-counter combinations. The result isn’t 16 evenly-graded timbres but a handful of distinct families — pure tones, buzzy/reedy tones, and noise. The ones you’ll reach for most:

Playing Music

Mon, 01 Jan 0001 00:00:00 +0000

Playing Music#

A steady tone is just six registers holding their values. Music is what happens when you change them on a schedule — and the schedule is the frame. A small routine, run once per frame, reads a song out of a data table and updates the audio registers; that routine is a sound engine, and even a tiny one carries a whole tune.

A song is a data table#

The simplest useful format is a list of (note, duration) pairs — which is exactly what the Song table in xmas/xmas.asm is: a note index and a length in frames, terminated by a zero: